Re: create index on a jsonb timestamp field?

On 05/18/2019 8:17 pm, Tom Lane wrote:
> Larry Rosenman <ler@lerctr.org> writes:
>> when I try to create an index on the query_time field of the json
>> structure I get:
>> ler=# create index dns_query_time_idx on dns_query(((data -> 'message'
>> ->> 'query_time')::text::timestamptz));
>> ERROR:  functions in index expression must be marked IMMUTABLE
> 
> Yeah, because the timestamptz input function has dependencies on
> both the datestyle and timezone GUCs.  Given that your input is
> ISO-format with explicit time zone, you don't really care about
> either of those things, but the mutability check doesn't know that.
> 
>> Is there any easy way to do this?  Or, what would the experts 
>> recommend
>> here?
> 
> The sanest way to deal with this IMO is to make a column containing
> the extracted timestamp, which you could maintain with a trigger,
> and then index that.  You could alternatively make a custom function
> that you (mis?)label as immutable, but your queries would have to
> use that same function in order to get matched to the index, so
> I dunno about that being a user-friendly approach.

This is what I wound up figuring out between my original post and yours,
and it works great.


> 
> BTW, I'd had the idea that the GENERATED option in PG v13 would allow
> setting up this sort of case without bothering with a handwritten 
> trigger,
> but it seems not:
> 
> regression=# create table foo(data jsonb, ts timestamptz GENERATED
> ALWAYS AS ((data->>'ts')::timestamptz) stored);
> psql: ERROR:  generation expression is not immutable
> 
> I wonder if that's really necessary to insist on?

Good question.  Is that something the project is going to look into?

> 
>             regards, tom lane

-- 
Larry Rosenman                     http://www.lerctr.org/~ler
Phone: +1 214-642-9640                 E-Mail: ler@lerctr.org
US Mail: 5708 Sabbia Dr, Round Rock, TX 78665-2106